It is becoming increasingly common for mobile telephones in particular to be provided with a digital video recording device. Zoom lenses are used in order to expand the photographic capabilities. Due to the limited space available in a mobile telephone, it is necessary to design appropriate zoom lenses in greatly miniaturized form.
In a basic configuration, such miniature zoom lenses are composed of an optical lens group having negative refractive power and an optical lens group having positive refractive power. In their simplest embodiment, a front lens group having negative refractive power and a lens group, situated behind same in the direction of light incidence, having positive refractive power are used. In order to generate an actual image in the image plane, the zoom lens formed in this manner must have an overall positive refractive power.
The primary function of a zoom lens is to change its overall refractive power, which according to the known principles of paraxial imaging allows a large object section, and therefore a large total object angle, to be detected at high refractive power, and allows a small object section, and therefore a small total object angle, to be detected at low refractive power. The focal length of a lens is equal to the reciprocal of its refractive power. A change in the system focal length represents the primary function of a zoom lens, and according to the known principles of paraxial imaging allows a large object section, i.e., a large total object angle 2w, at a short focal length (WA position), and allows a small value 2w at a long system focal length (telephoto position).
In a two-group optical system, when the first lens group (front lens group) has a negative focal length f′1 and the second lens group (rear lens group) has a positive focal length f′2, a change in the distance between the two lens groups results in a change in the overall focal length f′ of the system. If the imaging plane is to be kept unchanged at the location, the zoom action must be correspondingly distributed over both lens groups. For a change in the focal length of the system from f′ to f′+Δf′, the second lens group (variator) must be displaced from its starting position by an increment Δ2=(f′2/f′1)*Δf′, and the first lens group (compensator) must be correspondingly displaced as follows:Δ1=(f′2/f′1)*Δf′*[1−(f′21/f′(f′+Δf′)],where f′1, f′2 are the focal lengths of the first and the second lens group, respectively.
The discussion of both conditional equations for Δ1 and Δ2 shows that for a change in the system focal length f′ by Δf′, the displacement of the rear lens group (second lens group) is linear, and the displacement of the front lens group (first lens group) is nonlinear.
Corresponding zoom lenses are used not only in mobile telephones, but also in minicomputers, for example, in particular, in so-called personal digital assistants (PDAs). Such a zoom lens is known from EP 1 901 104 A1, for example.
Significant space limitations exist in particular when such a zoom lens is used in a mobile telephone. For this reason, a so-called periscope design is usually preferred, in which the part of the zoom lens which is moved during a zoom action is installed offset by 90° with respect to the front lens group. The beam path is deflected using a 90° prism (deflecting prism). A corresponding system is known from U.S. Pat. No. 7,312,931, for example.
Due to the limited installation space, corresponding miniature zoom lenses generally have a relatively low zoom factor, which is calculated from the ratio of the longest to the shortest focal length. To allow a low-aberration image to be obtained at the same time, the optical strain of the system must not exceed a specified value in any zoom position, i.e., at any of the available focal lengths. A first feature of the strain of the optical system may be the focal length of the individual lens groups. If this focal length is not too small, the geometric shape of the design elements, in particular lenses, in the optical lens group used may be satisfactorily adapted to the overall correction target of the zoom lens. However, in order to achieve a specified zoom factor, longer focal lengths require a regulating path which does not drop below a minimum value. If the adjustment space in the direction of the optical axis is too small, the refractive powers of the optical lens groups must be particularly large, resulting in considerable difficulties in the optical correction of the individual lens groups. One remedy is to form the individual optical lens groups from a fairly large number of components. However, installation space for this purpose is frequently lacking.
Due to the limited installation space, corresponding miniature zoom lenses may therefore be based only on relatively simple optical approaches, in particular a small number of lenses which have numerous aspherical surfaces for achieving the necessary imaging characteristics of the lens. US 2004/0257671 discloses a miniature zoom lens having a zoom factor of approximately 2.
In a transition to higher zoom factors >2, it has been shown that a three-group zoom structure is advantageous in which three optical lens groups are used to achieve a change in the focal length. In addition to stationary optical lens groups, such zoom lenses have a variator and two compensators, the function of the variator being to change the focal length, while the function of the compensator or compensators is to keep the location of the image generation constant as the focal length changes. Corresponding zoom lenses are known from US 2005/0105192, U.S. Pat. No. 7,315,422, U.S. Pat. No. 6,924,939, and US 2008/0062531, for example.